94 THE HUMAN PLACENTA. 



their formation the villi of the chorion consist of nucleated cells, bounded 

 externally by a fine transparent membrane, arid are immersed in the 

 substance of the cellular decidua reflexa, from which they absorb nutri- 

 ment for the embryo. When, however, the embryo has attained a 

 certain stage of development, at which the allantois containing the umbi- 

 lical vessels arrives at the internal surface of the chorion, and at which 

 blood-vessels are also formed within the villi * and communicate with the 

 umbilical vessels, the villi at that part which is to be converted into 

 the placenta, increase in number and in size, and come into relation with 

 the vascular system of the mother. At this period each villus contains 

 one or more loops of blood-vessels, its cells are diminished in quantity, 

 but a few are always found at its terminal extremity, and it still possesses 

 its external investing membrane. Meanwhile the vessels of the decidua 

 vera, or highly-developed mucous membrane of the uterus, enlarge, and 

 "assume the appearance of sinuses encroaching on the space formerly 

 occupied by the cellular decidua, in the midst of which the villi of the 

 chorion are imbedded. This increase in the calibre of the decidual capil- 

 laries goes on to such an extent that finally the villi are completely bound 

 up or covered by the membrane which constitutes the walls of the vessels, 

 this membrane following the contours of all the villi, and even passing to 

 a certain extent over the branches and stems of the tufts." In this 

 manner, as was shewn by Dr. J. Reid, Weber, and other observers,! the 

 tufts and villi of the foetal portion of the placenta are completely en- 

 sheathed by the lining membrane of the vascular system of the mother. 

 Between this membrane of the enlarged maternal vessels, and the mem- 

 brane of each villus, there still remains a layer of nucleated cells of the 

 decidua, (fig. 1 7 6), which the enlarging vessels have carried before them. 

 These cells, at certain parts of the circumference of the villus, are 

 grouped together in greater quantities, and appear to be passing off from 

 a spot in the centre of the mass (c) ; each such group constitutes what 

 Professor Goodsir calls a germinal spot, or nutritive centre, whose office 

 appears to be to supply a constant succession of new cells in the place of 

 those which are rapidly disappearing in the performance of the functions of 

 the villus. 



It appears, therefore, that, at the villi of the placental tufts, where the foetal 

 and maternal portions of the placenta are brought into close relation with 

 each other, the blood in the vessels of the mother is separated from that in 

 the vessels of the foetus by the intervention of two distinct sets of nucleated 



* This formation of blood-vessels within the villi appears to take place quite independently 

 of any communication with the vessels of the embyro, with which they only subsequently 

 unite (Bischoff, Entwick. der Saugeth. und des Menschen, p. 127) ; it is probably effected 

 through the transformation of the cells of the villi, in a manner which will be pointed out 

 hereafter when speaking of the mode of development of blood-vessels generally. 



f Muller's Physiology, p. 1606. See also Weber's Zusatze der Geschlechtsorgane 3 p. 41- 



